This invention relates to the solution polymerization of ethylene in two reactors using a catalyst having a phosphinimine ligand.
The use of so-called xe2x80x9csingle-sitexe2x80x9d catalysts such as metallocene catalysts to prepare polyethylene having a narrow molecular weight distribution is well known. In addition, xe2x80x9clinear low density polyethylenexe2x80x9d (or xe2x80x9cLLDPExe2x80x9d, a copolymer of ethylene and a higher alpha olefin) prepared with such catalysts typically exhibits a very uniform composition distribution (i.e. the comonomer is very uniformly distributed within the polymer chains). The combination of narrow molecular weight distribution and uniform composition distribution distinguishes these polymers from xe2x80x9cconventionalxe2x80x9d LLDPE which is commercially manufactured with a Ziegler Natta catalyst or a chromium catalyst. In particular, the conventional LLDPE products have a broad molecular weight distribution and a broad composition distribution. These compositional differences are manifested in the form of differences in the physical properties of the two types of LLDPE polymers. Most notably, LLDPE prepared with a single site catalyst has improved dart impact strength and optical properties in comparison to xe2x80x9cconventionalxe2x80x9d LLDPE. However, the xe2x80x9cconventionalxe2x80x9d LLDPE is usually easier to xe2x80x9cprocessxe2x80x9d in its existing mixing and extrusion equipment. Accordingly, it would be highly desirable to prepare LLDPE products which possess the improved physical properties offered by single site catalysts and also exhibit processability characteristics which are similar to those of conventional LLDPE.
One approach which has been used to achieve this object is the use of mixed catalyst systems in a single reactor. For example, U.S. Pat. No. 4,530,914 (Ewen et al, to Exxon) teaches the use of two different metallocenes and U.S. Pat. No. 4,701,432 (Welborm, to Exxon) teaches the use of a supported catalyst prepared with a metallocene catalyst and a Ziegler Natta catalyst. Many others have subsequently attempted to use similar mixed catalyst systems as described in U.S. Pats. No. 5,767,031; 5,594,078; 5,648,428; 4,659,685; 5,145,818; 5,395,810; and 5,614,456.
However, the use of xe2x80x9cmixedxe2x80x9d catalyst systems is generally associated with operability problems. For example, the use of two catalysts on a single support (as taught by Welborm in U.S. Pat. No. 4,701,432) may be associated with a reduced degree of process control flexibility (e.g. If the polymerization reaction is not proceeding as desired when using such a catalyst system, then it is difficult to establish which corrective action should be taken as the corrective action will typically have a different effect on each of the two different catalyst components). Moreover, the two different catalyst/cocatalyst systems may interfere with one anotherxe2x80x94for example, the organoaluminum component which is often used in Ziegler Natta or chromium catalyst systems may xe2x80x9cpoisonxe2x80x9d a metallocene catalyst.
Another alternative is to use two different metallocene catalysts in two different polymerization reactors. However, process control problems relating to interactions between the two different catalysts might also be anticipated in such a process. Accordingly, a xe2x80x9cdual reactorxe2x80x9d process which mitigates some of these problems would be a useful addition to the art.
The present invention provides a medium pressure solution polymerization process characterized by:
A) polymerizing ethylene, optionally with one or more C3-12 alpha olefins, in solvent in a first stirred polymerization reactor at a temperature of from 80 to 200xc2x0 C. and a pressure of from 1500 to 5000 pounds per square inch gauge (psi) in the presence of (a) a catalyst which is an organometallic complex of a group 3, 4 or 5 metal, characterized by having at least one phosphinimine ligand; and (b) a cocatalyst which contains an alumoxane; then
B) passing said first polymer solution into a second stirred polymerization reactor and polymerizing ethylene, optionally with one or more C3-12 alpha olefins, in said second stirred polymerization reactor in the presence of (a) a catalyst which is an organometallic complex of a group 3, 4 or 5 metal, characterized by having at least one phosphinimine ligand; and (b) a cocatalyst which contains an ionic activator.
Thus, the process of the present invention requires two solution polymerization reactors and a catalyst having a phosphinimine ligand (xe2x80x9cphosphinimine catalystxe2x80x9d).
Preferred catalysts are titanium complexes which contain one cyclopentadienyl ligand, one phosphinimine ligand and two chloride ligands. The same phosphinimine catalyst may be used in both reactors or, alternatively, a different type of phosphinimine catalyst may be used in the two reactors. It is preferred to use the same catalyst in both reactors. Preferred co-catalysts are selected from a boron-containing xe2x80x9cionic activatorsxe2x80x9d and alumoxanes.
Thus, in the present process, the cocatalyst system used in the first reactor must be different from the cocatalyst system used in the second reactor. In addition, it is preferred that polymerization temperature in the second reactor is different than the polymerization temperature of the first reactor.
Most preferably, the second polymerization reactor is operated at a higher temperature than the first (ideally at least 25xc2x0 C. higher than the first).
Certain LLDPE polymers produced according to the preferred process of this invention exhibit an outstanding balance of physical properties, optical properties and xe2x80x9cprocessabilityxe2x80x9d. As will be recognized by those skilled in the art, this balance of characteristics is highly desirable for the production of LLDPE film. Thus, the present invention also provides a LLDPE film having a dart impact strength as determined by ASTM D-1709 of greater than 700 grams/mil, a haze as determined by ASTM D-1003 of less that 6%, a 45xc2x0 gloss as determined by ASTM D-2457 of greater that 65% and a machine direction tear resistance as determined by ASTM D-1922 of greater than 300 grams/mil.
The catalyst used in the process of this invention is an organometallic complex of a group 3, 4 or 5 metal which is characterized by having at least one phosphinimine ligand (where the term phosphinimine is defined in section 1.2 below).
Any such organometallic having a phosphinimine ligand which displays catalytic activity for ethylene polymerization may be employed. Preferred catalysts are defined by the formula: 
wherein M is a transition metal selected from Ti, Hf and Zr (as described in section 1.1 below); PI is a phosphinimine ligand (as described in section 1.2 below); L is a monanionic ligand which is a cyclopentadienyl-type ligand or a bulky heteroatom ligand (as described in section 1.3 below); X is an activatable ligand which is most preferably a simple monanionic ligand such as alkyl or a halide (as described in section 1.4 below); M is 1 or 2, n is 0 or 1, and p is fixed by the valence of the metal M.
The most preferred first catalysts are group 4 metal complexes in the highest oxidation state. For example, a preferred catalyst may be a bis(phosphinimine) dichloride complex of titanium, zirconium or hafnium. However, it is preferred that the first catalyst contain one phosphinimine ligand, one xe2x80x9cLxe2x80x9d ligand (which is most preferably a cyclopentadienyl-type ligand) and two xe2x80x9cXxe2x80x9d ligands (which are preferably both chloride).
1.1 Metals
The catalyst is an organometallic complex of a group 3, 4 or 5 metal (where the numbers refer to columns in the Periodic Table of the Elements using IUPAC nomenclature). The preferred metals are from group 4, (especially titanium, hafnium or zirconium) with titanium being most preferred.
1.2 Phosphinimine Ligand
The first catalyst must contain a phosphinimine ligand which is covalently bonded to the metal. This ligand is defined by the formula: 
wherein each R1 is independently selected from the group consisting of a hydrogen atom, a halogen atom, C1-20 hydrocarbyl radicals which are unsubstituted by or further substituted by a halogen atom, a C1-8 alkoxy radical, a C6-10 aryl or aryloxy radical, an amido radical, a silyl radical of the formula:
xe2x80x94Sixe2x80x94(R2)3
wherein each R2 is independently selected from the group consisting of hydrogen, a C1-8 alkyl or alkoxy radical, C6-10 aryl or aryloxy radicals, and a germanyl radical of the formula:
Gexe2x80x94(R2)3
wherein R2 is as defined above.
The preferred phosphinimines are those in which each R1 is a hydrocarbyl radical. A particularly preferred phosphinimine is tri-(tertiary butyl)phosphinimine (i.e. where each R1 is a tertiary butyl group).
1.3 Ligand L
Preferred first catalysts are group 4 organometallic complexes which contain one phosphinimine ligand (as described in section 1.2 above) and one ligand L (as described in sections 1.3.1 to 1.3.6) which is either a cyclopentadienyl-type ligand or a heteroligand.
1.3.1 Cyclopentadienyl-type Ligands
As used herein, the term cyclopentadienyl-type ligand is meant to convey its conventional meaning, namely a ligand having a five carbon ring which is bonded to the metal via eta-5 bonding. Thus, the term xe2x80x9ccyclopentadienyl-typexe2x80x9d includes unsubstituted cyclopentadienyl, substituted cyclopentadienyl, unsubstituted indenyl, substituted indenyl, unsubstituted fluorenyl and substituted fluorenyl. An exemplary list of substituents for a cyclopentadienyl ligand includes the group consisting of C1-10 hydrocarbyl radical (which hydrocarbyl substituents are unsubstituted or further substituted); a halogen atom, C1-8 alkoxy radical, a C1-10 aryl or aryloxy radical; an amido radical which is unsubstituted or substituted by up to two C1-8 alkyl radicals; a phosphido radical which is unsubstituted or substituted by up to two C1-8 alkyl radicals; silyl radicals of the formula xe2x80x94Sixe2x80x94(R)3 wherein each R is independently selected from the group consisting of hydrogen, a C1-8 alkyl or alkoxy radical C6-10 aryl or aryloxy radicals; germanyl radicals of the formula Gexe2x80x94(R)3 wherein R is as defined directly above.
1.3.2 Heteroligand
As used herein, the term xe2x80x9cheteroligandxe2x80x9d refers to a ligand which contains at least one heteroatom selected from the group consisting of boron, nitrogen, oxygen, phosphorus or sulfur. The heteroligand may be sigma or pi-bonded to the metal. Exemplary heteroligands are described in sections 1.3.2.1 to 1.3.2.6 below.
1.3.2.1 Ketimide Ligands
As used herein, the term xe2x80x9cketimide ligandxe2x80x9d refers to a ligand which: (a) is bonded to the transition metal via a metal-nitrogen atom bond; (b) has a single substituent on the nitrogen atom, (where this single substituent is a carbon atom which is doubly bonded to the N atom); and (c) has two substituents (Sub 1 and Sub 2, described below) which are bonded to the carbon atom. 
The substituents xe2x80x9cSub 1 and Sub 2xe2x80x9d may be the same or different. Exemplary substituents include hydrocarbyls having from 1 to 20 carbon atoms; silyl groups, amido groups and phosphido groups. For reasons of cost and convenience it is preferred that these substituents both be hydrocarbyls, especially simple alkyls and most preferably tertiary butyl.
1.3.2.2 Silicone-Containing Heteroligands
These ligands are defined by the formula:
xe2x80x94(xcexc)SiRxRyRz
where thexe2x80x94denotes a bond to the transition metal and xcexc is sulfur or oxygen.
The substituents on the Si atom, namely Rx, Ry and Rz are required in order to satisfy the bonding orbital of the Si atom. The use of any particular substituent Rx, Ry or Rz is not especially important to the success of this invention. It is preferred that each of Rx, Ry and Rz is a C1-2 hydrocarbyl group (i.e. methyl or ethyl) simply because such materials are readily synthesized from commercially available materials).
1.3.2.3 Amido Ligands
The term xe2x80x9camidoxe2x80x9d is meant to convey its broad, conventional meaning. Thus, these ligands are characterized by (a) a metal-nitrogen bond, and (b) the presence of two substituents (which are typically simple alkyl or silyl groups) on the nitrogen atom.
1.3.2.4 Alkoxy Ligands
The term xe2x80x9calkoxyxe2x80x9d is also intended to convey its conventional meaning. Thus these ligands are characterized by (a) a metal oxygen bond, and (b) the presence of a hydrocarbyl group bonded to the oxygen atom. The hydrocarbyl group may be a ring structure and/or substituted (e.g. 2,6 di-tertiary butyl phenoxy).
1.3.2.5 Boron Heterocyclic Ligands
These ligands are characterized by the presence of a boron atom in a closed ring ligand. This definition includes heterocyclic ligands which also contain a nitrogen atom in the ring. These ligands are well known to those skilled in the art of olefin polymerization and are fully described in the literature (see, for example, U.S. Pat. Nos. 5,637,659; 5,554,775 and the references cited therein).
1.3.2.6 Phosphole Ligands
The term xe2x80x9cphospholexe2x80x9d is also meant to convey its conventional meaning. xe2x80x9cPhospholesxe2x80x9d are cyclic dienyl structures having four carbon atoms and one phosphorus atom in the closed ring. The simplest phosphole is C4PH4 (which is analogous to cyclopentadiene with one carbon in the ring being replaced by phosphorus). The phosphole ligands may be substituted with, for example, C1-20 hydrocarbyl radicals (which may, optionally, contain halogen substituents); phosphido radicals; amido radicals; silyl or alkoxy radicals. Phosphole ligands are also well known to those skilled in the art of olefin polymerization and are described as such in U.S. Pat. No. 5,434,116 (Sone, to Tosoh).
1.4 Activatable Ligand
The term xe2x80x9cactivatable ligandxe2x80x9d refers to a ligand which may be activated by a cocatalyst, (also referred to as an xe2x80x9cactivatorxe2x80x9d), to facilitate olefin polymerization. Exemplary activatable ligands are independently selected from the group consisting of a hydrogen atom, a halogen atom, a C1-10 hydrocarbyl radical, a C1-10 alkoxy radical, a C5-10 aryl oxide radical; each of which said hydrocarbyl, alkoxy, and aryl oxide radicals may be unsubstituted by or further substituted by a halogen atom, a C1-8 alkyl radical, a C1-8 alkoxy radical, a C6-10 aryl or aryloxy radical, an amido radical which is unsubstituted or substituted by up to two C1-8 alkyl radicals; a phosphido radical which is unsubstituted or substituted by up to two C1-8 alkyl radicals.
The number of activatable ligands depends upon the valency of the metal and the valency of the activatable ligand. The preferred catalyst metals are group 4 metals in their highest oxidation state (i.e. 4+) and the preferred activatable ligands are monoanionic (such as a halidexe2x80x94especially chloride or a alkylxe2x80x94especially methyl). Thus, the preferred catalyst contain a phosphinimine ligand, a cyclopentadienyl ligand and two chloride (or methyl) ligands bonded to the group 4 metal. In some instances, the metal of the catalyst component may not be in the highest oxidation state. For example, a titanium (III) component would contain only one activatable ligand.
1.5 Summary Description of Preferred Catalyst
As previously noted, the preferred catalyst is a group 4 organometallic complex in its highest oxidation state having a phosphinimine ligand, a cyclopentadienyl-type ligand and two activatable ligands. These requirements may be concisely described using the following formula for the preferred catalyst: 
wherein: (a) M is a metal selected from Ti, Hf and Zr; (b) PI is a phosphinimine ligand defined by the formula: 
wherein each R1 is independently selected from the group consisting of a hydrogen atom, a halogen atom, C1-20 hydrocarbyl radicals which are unsubstituted by or further substituted by a halogen atom, a C1-8 alkoxy radical, a C6-10 aryl or aryloxy radical, an amido radical, a silyl radical of the formula:
xe2x80x83xe2x80x94Sixe2x80x94(R2)3
wherein each R2 is independently selected from the group consisting of hydrogen, a C1-8 alkyl or alkoxy radical, C6-10 aryl or aryloxy radicals, and a germanyl radical of the formula:
Gexe2x80x94(R2)3
wherein R2 is as defined above; (c) L is a ligand selected from the group consisting of cyclopentadienyl, substituted cyclopentadienyl, indenyl, substituted indenyl, fluorenyl, substituted fluorenyl; (d) X is an activatable ligand, and wherein: m is 1, n is 1 and p is 2.
The catalyst components described in part 1 above are used in combination with at least one cocatalyst (or xe2x80x9cactivatorxe2x80x9d) to form an active catalyst system for olefin polymerization as described in more detail in sections 2.1, 2.2 and 2.3 below.
2.1 Alumoxanes
The alumoxane may be of the formula:
(R4)2AlO(R4AlO)mAl(R4)2
wherein each R4 is independently selected from the group consisting of C1-20 hydrocarbyl radicals and m is from 0 to 50, preferably R4 is a C1-4 alkyl radical and m is from 5 to 30. Methylalumoxane (or xe2x80x9cMAOxe2x80x9d) in which each R is methyl is the preferred alumoxane.
Alumoxanes are well known as cocatalysts, particularly for metallocene-type catalysts. Alumoxanes are also readily available articles of commerce.
The use of an alumoxane cocatalyst generally requires a molar ratio of aluminum to the transition metal in the catalyst from 20:1 to 1000:1. Preferred ratios are from 50:1 to 250:1.
2.2 xe2x80x9cIonic Activatorsxe2x80x9d Cocatalysts
So-called xe2x80x9cionic activatorsxe2x80x9d are also well known for metallocene catalysts. See, for example, U.S. Pat No. 5,198,401 (Hlatky and Turner) and U.S. Pat No. 5,132,380 (Stevens and Neithamer).
Whilst not wishing to be bound by any theory, it is thought by those skilled in the art that xe2x80x9cionic activatorsxe2x80x9d initially cause the abstraction of one or more of the activatable ligands in a manner which ionizes the catalyst into a cation, then provides a bulky, labile, non-coordinating anion which stabilizes the catalyst in a cationic form. The bulky, non-coordinating anion permits olefin polymerization to proceed at the cationic catalyst center (presumably because the non-coordinating anion is sufficiently labile to be displaced by monomer which coordinates to the catalyst. Preferred ionic activators are boron-containing ionic activators described in (i)-(iii) below:
(i) compounds of the formula [R5]+[B(R7)4]xe2x88x92 wherein B is a boron atom, R5 is a aromatic hydrocarbyl (e.g. triphenyl methyl cation) and each R7 is independently selected from the group consisting of phenyl radicals which are unsubstituted or substituted with from 3 to 5 substituents selected from the group consisting of a fluorine atom, a C1-4 alkyl or alkoxy radical which is unsubstituted or substituted by a fluorine atom; and a silyl radical of the formula xe2x80x94Sixe2x80x94(R9)3; wherein each R9 is independently selected from the group consisting of a hydrogen atom and a C1-4 alkyl radical; and
(ii) compounds of the formula [(R8)tZH]+[B(R7)4]xe2x88x92 wherein B is a boron atom, H is a hydrogen atom, Z is a nitrogen atom or phosphorus atom, t is 2 or 3 and R8 is selected from the group consisting of C1-8 alkyl radicals, a phenyl radical which is unsubstituted or substituted by up to three C1-4 alkyl radicals, or one R8 taken together with the nitrogen atom may form an anilinium radical and R7 is as defined above; and
(iii) compounds of the formula B(R7)3 wherein R7 is as defined above.
In the above compounds preferably R7 is a pentafluorophenyl radical, and R5 is a triphenylmethyl cation, Z is a nitrogen atom and R8 is a C1-4 alkyl radical or R8 taken together with the nitrogen atom forms an anilinium radical which is substituted by two C1-4 alkyl radicals.
The xe2x80x9cionic activatorxe2x80x9d may abstract one or more activatable ligands so as to ionize the catalyst center into a cation but not to covalently bond with the catalyst and to provide sufficient distance between the catalyst and the ionizing activator to permit a polymerizable olefin to enter the resulting active site.
Examples of ionic activators include:
triethylammonium tetra(phenyl)boron,
tripropylammonium tetra(phenyl)boron,
tri(n-butyl)ammonium tetra(phenyl)boron,
trimethylammonium tetra(p-tolyl)boron,
trimethylammonium tetra(o-tolyl)boron,
tributylammonium tetra(pentafluorophenyl)boron,
tripropylammonium tetra(o,p-dimethylphenyl)boron,
tributylammonium tetra(m,m-dimethylphenyl)boron,
tributylammonium tetra(p-trifluoromethylphenyl)boron,
tributylammonium tetra(pentafluorophenyl)boron,
tri(n-butyl)ammonium tetra(o-tolyl)boron,
N,N-dimethylanilinium tetra(phenyl)boron,
N,N-dimethylanilinium tetra(phenyl)boron,
N,N-diethylanilinium tetra(phenyl)n-butylboron,
N,N-2,4,6-pentamethylanilinium tetra(phenyl)boron,
di-(isopropyl)ammonium tetra(pentafluorophenyl)boron,
dicyclohexylammonium tetra(phenyl)boron,
triphenylphosphonium tetra(phenyl)boron,
tri(methylphenyl )phosphonium tetra(phenyl)boron,
tri(dimethylphenyl)phosphonium tetra(phenyl)boron,
tropillium tetrakispentafluorophenyl borate,
triphenylmethylium tetrakispentafluorophenyl borate,
benzene(diazonium)tetrakispentafluorophenyl borate,
tropillium phenyltrispentafluorophenyl borate,
triphenylmethylium phenyltrispentafluorophenyl borate,
benzene(diazonium)phenyltrispentafluorophenyl borate,
tropillium tetrakis(2,3,5,6-tetrafluorophenyl)borate,
triphenylmethylium tetrakis(2,3,5,6-tetrafluorophenyl)borate,
benzene (diazonium)tetrakis(3,4,5-trifluorophenyl)borate,
tropillium tetrakis(3,4,5-trifluorophenyl)borate,
benzene(diazonium)tetrakis(3,4,5-trifluorophenyl)borate,
tropillium tetrakis(1,2,2-trifluoroethenyl)borate,
triphenylmethylium tetrakis(1,2,2-trifluoroethenyl)borate, benzene(diazonium)tetrakis(1,2,2-trifluoroethenyl)borate,
tropillium tetrakis(2,3,4,5-tetrafluorophenyl)borate,
triphenylmethylium tetrakis(2,3,4,5-tetrafluorophenyl)borate, and
benzene(diazonium)tetrakis(2,3,4,5-tetrafluorophenyl)borate.
Readily commercially available ionic activators include:
N,N-dimethylaniliumtetrakispentafluorophenyl borate, triphenylmethylium tetrakispentafluorophenyl borate, and trispentafluorophenyl borane.
2.3 Cocatalyst Systems
An alumoxane must be used in the first reactor and an ionic activator must be used in the second reactor. However, it is permissible to use both of an alumoxane and an ionic activator in either reactor, so the term xe2x80x9ccocatalyst systemxe2x80x9d is used to convey this option. In particular, it is preferred to use alumoxane in both reactors as alumoxanes are generally regarded as very good xe2x80x9cpoison scavengersxe2x80x9d (i.e. the alumoxanes are thought to mitigate the deleterious effects of contaminants which may be present in the reactor).
The cocatalyst systems used in the two reactors are different. If a boron-containing xe2x80x9cionic activatorxe2x80x9d and alumoxane are used in both reactors, it is possible to satisfy the requirement that different cocatalyst systems in the two reactors by using different mole ratios of the two catalysts in the two reactors (as is illustrated in the Examples). We have discovered that the use of different cocatalyst systems in the reactors provides process control options which may be readily used by persons skilled in the art to vary the MWD of polymers produced by the inventive process.
3. Description of Dual Reactor Solution Polymerization Process
Solution processes for the (co)polymerization of ethylene are well known in the art. These processes are conducted in the presence of an inert hydrocarbon solvent typically a C5-12 hydrocarbon which may be unsubstituted or substituted by a C1-4 alkyl group, such as pentane, methyl pentane, hexane, heptane, octane, cyclohexane, methylcyclohexane and hydrogenated naphtha. An example of a suitable solvent which is commercially available is xe2x80x9cIsopar Exe2x80x9d (C8-12 aliphatic solvent, Exxon Chemical Co.).
The solution polymerization process of this invention must use at least two polymerization reactors. The first polymerization reactor preferably operates at a lower temperature (xe2x80x9ccold reactorxe2x80x9d) using a xe2x80x9cphosphinimine catalystxe2x80x9d described in Part 1 above.
The polymerization temperature in the first reactor is from about 80xc2x0 C. to about 180xc2x0 C. (preferably from about 120xc2x0 C. to 160xc2x0 C.) and the hot reactor is preferably operated at a higher temperature (up to about 220xc2x0 C.). The most preferred reaction process is a xe2x80x9cmedium pressure processxe2x80x9d, meaning that the pressure in each reactor is preferably less than about 6,000 psi (about 42,000 kilopascals or kPa), most preferably from about 2,000 psi to 3,000 psi (about 14,000-22,000 kPa)
Suitable monomers for copolymerization with ethylene include C3-20 mono- and di-olefins. Preferred comonomers include C3-12 alpha olefins which are unsubstituted or substituted by up to two C1-6 alkyl radicals, C8-12 vinyl aromatic monomers which are unsubstituted or substituted by up to two substituents selected from the group consisting of C1-4 alkyl radicals, C4-12 straight chained or cyclic diolefins which are unsubstituted or substituted by a C1-4 alkyl radical. Illustrative non-limiting examples of such alpha-olefins are one or more of propylene, 1-butene, 1-pentene, 1-hexene, 1-octene and 1-decene, styrene, alpha methyl styrene, and the constrained-ring cyclic olefins such as cyclobutene, cyclopentene, dicyclopentadiene norbornene, alkyl-substituted norbornes, alkenyl-substituted norbornes and the like (e.g. 5-methylene-2-norbornene and 5-ethylidene-2-norbornene, bicyclo-(2,2,1 )-hepta-2,5-diene).
The polyethylene polymers which may be prepared in accordance with the present invention are LLDPE""s which typically comprise not less than 60, preferably not less than 75 weight % of ethylene and the balance one or more C4-10 alpha olefins, preferably selected from the group consisting of 1-butene, 1-hexene and 1-octene. The polyethylene prepared in accordance with the present invention may be LLDPE having a density from about 0.910 to 0.935 g/cc or (linear) high density polyethylene having a density above 0.935 g/cc. The present invention might also be useful to prepare polyethylene having a density below 0.910 g/ccxe2x80x94the so-called very low and ultra low density polyethylenes.
Generally the alpha olefin may be present in an amount from about 3 to 30 weight %, preferably from about 4 to 25 weight %.
The present invention may also be used to prepare co- and ter-polymers of ethylene, propylene and optionally one or more diene monomers. Generally, such polymers will contain about 50 to about 75 weight % ethylene, preferably about 50 to 60 weight % ethylene and correspondingly from 50 to 25 weight % of propylene. A portion of the monomers, typically the propylene monomer, may be replaced by a conjugated diolefin. The diolefin may be present in amounts up to 10 weight % of the polymer although typically is present in amounts from about 3 to 5 weight %. The resulting polymer may have a composition comprising from 40 to 75 weight % of ethylene, from 50 to 15 weight % of propylene and up to 10 weight % of a diene monomer to provide 100 weight % of the polymer. Preferred but not limiting examples of the dienes are dicyclopentadiene, 1,4-hexadiene, 5-methylene-2-norbornene, 5-ethylidene-2-norbornene and 5-vinyl-2-norbornene, especially 5-ethylidene-2-norbornene and 1,4-hexadiene.
The monomers are dissolved/dispersed in the solvent either prior to being fed to the first reactor (or for gaseous monomers the monomer may be fed to the reactor so that it will dissolve in the reaction mixture). Prior to mixing, the solvent and monomers are generally purified to remove potential catalyst poisons such as water, oxygen or metal impurities. The feedstock purification follows standard practices in the art, e.g. molecular _ sieves, alumina beds and oxygen removal catalysts are used for the purification of monomers. The solvent itself as well (e.g. methyl pentane, cyclohexane, hexane or toluene) is preferably treated in a similar manner.
The feedstock may be heated or cooled prior to feeding to the first reactor. Additional monomers and solvent may be added to the second reactor, and it may be heated or cooled.
Generally, the catalyst components may be premixed in the solvent for the reaction or fed as separate streams to each reactor. In some instances premixing it may be desirable to provide a reaction time for the catalyst components prior to entering the reaction. Such an xe2x80x9cin line mixingxe2x80x9d technique is described in a number of patents in the name of DuPont Canada Inc (e.g. U.S. Pat No. 5,589,555, issued Dec. 31, 1996).
The residence time in each reactor will depend on the design and the capacity of the reactor. Generally the reactors should be operated under conditions to achieve a thorough mixing of the reactants. In addition, it is preferred that from 20 to 60 weight % of the final polymer is polymerized in the first reactor, with the balance being polymerized in the second reactor. On leaving the reactor system the solvent is removed and the resulting polymer is finished in a conventional manner.
In a highly preferred embodiment, the first polymerization reactor has a smaller volume than the second polymerization reactor. In addition, the first polymerization reactor is preferably operated at a colder temperature than the second reactor. Certain LLDPE polymers produced under these highly preferred conditions have outstanding properties. In particular, the ethylene-octane type LLDPE polymers illustrated in the following examples have excellent impact strength and tear, whilst still retaining good optical properties (which are typically associated with LLDPE prepared using metallocene catalysts) and exhibiting good processability (often associated with LLDPE produced using conventional Ziegler Natta catalysts).